In vivo studies on the oncogenes sis, ras, erbB and TGFAlpha are producing needed data on the pathogenesis of tumor induction and cell transformation by these genes. Essential differences among these oncogenes are also being studied in parallel in vitro and in vivo experiments. Genetically engineered type C viruses incorporating each oncogene are usually administered to newborn mice. The ras-containing agents produce extremely early tumors (14 days), whereas the other oncogenes studied to date require several months. In addition, murine cells transfected and transformed in vitro are grafted to adult mice to study the progress of neoplasia characteristic for each oncogene. Human cells transformed by oncogenic viruses with and without chemical cocarcinogens are also tested for characteristic tumorigenicity.